System for manufacturing a group of head gimbal assemblies (HGAs)

ABSTRACT

A system and method for the production level screening of low flying magnetic heads in the manufacture of disk drive head disk assemblies (HDAs) is disclosed. A test disk is provided and has a plurality of bumps extending from at least one surface thereof to a predetermined height between 2 and 12 nanometers. The test disk is rotated to fly a head of a head gimbal assembly selected from the group adjacent the surface of the test disk. An interaction of the head with one or more of the plurality of bumps may be sensed and the head gimbal assembly may be screened out from the group in response to the sensing of the interaction.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.11/060,895, filed Feb. 18, 2005, which is incorporated by referenceherein in its entirety.

BACKGROUND OF THE INVENTION

The present invention relates, in general, to the field of rotating datastorage media. More particularly, the present invention relates to asystem and method for the production level screening of low flyingmagnetic heads in the manufacture of disk drive head disk assemblies(HDAs).

A partially cut-away, isometric illustration of a typical prior art diskdrive HDA 10 is shown in FIG. 1. The HDA 10 includes a number of disks12 which are rotated about a spindle 14 by means of a motor (not shown).An actuator motor 16 positions an arm 18 with respect to data tracks onthe surfaces of the disks 12. The actuator arm, in turn, positions asuspension 20 and head 22 which flies adjacent to the rotating surfacesof the disks 12.

A disk drive read/write head generally comprises a read/write transducerand a slider that includes an air bearing surface (ABS). The ABS allowsthe slider to “fly” adjacent the surface of a rotating disk due to thedevelopment of an air bearing between the disk surface and the ABS. Theslider is generally bonded to a thin metal arm, or suspension, thatholds the head in position above or beneath the rotating disks.Typically, the combination of a head and suspension is called a headgimbal assembly (HGA) and multiple HGAs may be stacked together to forma head stack assembly (HSA). Functionally, the arms and heads of the HSAare positioned with respect to the respective disk surfaces duringoperation by means of an actuator or servo mechanism.

As mentioned previously, during normal operation, the read/write head isseparated from the disk surface as it spins by a thin air bearing. Thesuspension serves to apply a force in a direction opposite to thepressure generated by the air bearing to maintain an equilibriumcondition in which the transducer is separated from the disk surface bya small controlled spacing, to enable the reading and writing of data.If the desired equilibrium condition is disturbed, for example byexcessive shock or vibration, or if the equilibrium condition is neverestablished, for example due to component manufacturing variances, thehead can crash into the disk surface. Not only can this damage the disksurface at that location, but debris from the crash site can causefurther problems throughout the HDA.

As the areal density of disk drives increases, heads are required to flylower and lower to the disk surface in order to read and write data.With current technology, this height can be below 0.5 micro-inches. Inactual head production, there is often a variation in the fly heights ofthe heads in a given product due to process specific tolerances.Heretofore, most approaches have attempted to control and reduce theflying height variations (sigma) on a given lot, and from lot to lot.The variations may also be reduced by improved suspension design (e.g.low stiffness), the ABS design, and other manufacturing process controlsthat reduce fly height sensitivity to process specific tolerances.

To account for fly height variation, the ABS must be designed to flyslightly higher than would otherwise be desirable. The fly heightresulting from an ABS design is typically determined by computermodeling and simulation based on the well-known Reynold's Equation.However such computer simulation results must be confirmed andcalibrated by experimental testing of fly height.

In this regard, certain patents illustrating the current state of theart in making and using calibration disks to enable the testing of theflying heights of certain heads in a test environment include: U.S. Pat.No. 5,528,922 issued Jun. 25, 1996 for: “Method of Making Disk Bumpswith Laser Pulses for Calibrating PZT Sliders”; U.S. Pat. No. 6,408,677issued Jun. 25, 2002 for: “Calibration Disk Having Discrete Bands ofComposite Roughness”; and U.S. Pat. No. 6,164,118 issued Dec. 26, 2000for: “Calibration Disk Having Discrete Bands of Calibration Zones”. Itshould be noted that the subject matter of these specific patents isdirected to testing (e.g., glide head calibration) in a test environmentand not a production level screening technique as disclosed herein.

SUMMARY OF THE INVENTION

Disclosed herein is a method for manufacturing a group of head gimbalassemblies. The method comprises the acts of providing a test diskhaving a plurality of bumps extending from at least one surface thereof,rotating the test disk to fly a head of a head gimbal assembly selectedfrom the group adjacent the at least one surface of the test disk,sensing an interaction of the head with one or more of the plurality ofbumps and screening out the head gimbal assembly selected from the groupin response to the sensing of the interaction. Further provided hereinis a system for implementing the aforedescribed method and a disk drivehead disk assembly including at least one of a group of head gimbalassemblies screened by the method.

BRIEF DESCRIPTION OF THE DRAWINGS

The aforementioned and other features and objects of the presentinvention and the manner of attaining them will become more apparent andthe invention itself will be best understood by reference to thefollowing description of exemplary embodiments taken in conjunction withthe accompanying drawings, wherein:

FIG. 1 is a isometric illustration of a partially cut-away,representative prior art disk drive head disk assembly (HDA);

FIG. 2A is a simplified representation of a disk drive read/write headslider assembly flying above the surface of a rotating disk as affixedto an associated suspension and test arm having, for example, anacoustic emission (AE) sensor associated therewith;

FIG. 2B is an enlarged view of a read/write head slider assembly asshown in the preceding figure illustrative of the head flying below theheight of predetermined height laser bumps associated with a DET testdisk media and which will generate an AE signal upon contact between thehead and the bump;

FIGS. 3A, 3B and 3C are graphical representations of exemplary flyheight distributions at mean fly heights (FH) of substantially 10.0 nm(0.40 micro-inches), 8.0 nm (0.32 micro-inches) and 6.5 nm (0.23micro-inches) respectively illustrating fly height and glide avalanche(GA) ranges at population frequencies of between 0.0 and 2000 whereinthe low flying heads are seen to touch as the mean fly height islowered;

FIG. 4 is a simplified production flow chart in accordance with thepresent invention illustrating the use of an on-line AE tester to enablethe screening of low flying heads prior to dynamic electrical test(DET);

FIG. 5 is a graphical illustration of an exemplary minimum head flyheight cut off (in micro inches) versus bump height (in Angstroms)assuming negligible air cushion effects in a representativeimplementation of the system and method of the present invention; and

FIG. 6 is a functional block diagram of an integrated head tester systemin accordance with the present invention illustrative of key componentsfor DET measurements and screening of low flying heads utilizing, forexample, AE signal detection.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

An embodiment of the present invention, as disclosed herein,advantageously provides a production technique that serves to identifyand remove the extreme low flying heads (e.g. the lower approximately2.0%-5.0% of the distribution) utilizing, for example, acoustic emission(AE) sensors and laser bumps on test disks. Removal of the lower flyingdevices in the fly height distribution effectively serves to reduce theincidence of head/disk contact, and hence, to improve the overallreliability of the drives ultimately produced.

A system and method of an embodiment of the present invention furtherprovides for production line monitoring of head fly height to enable thescreening out of very low flying heads which may cause HDA reliabilityproblems. The reliability of the resultant disk drives may besignificantly improved by reducing the head/disk interface problemscaused by low flying heads. Such problems may include, for example,lubricant degradation, read/write errors, debris generation and thelike.

Implementation of a system and method of an embodiment of the presentinvention may be effectuated by the inclusion of, for example, anacoustic emission sensor and associated amplifier into existing industrytest equipment and the provision of suitable media for dynamicelectrical testing (DET) with added laser bumps at one or more of theinner diameter (ID), middle diameter (MD) and/or outer diameter (OD) ofthe disk. The result is a test environment which may be implemented in amanner compatible with existing glide height testing for disks.

With reference additionally now to FIG. 2A, a simplified representationof a system 100 comprising a disk drive read/write head slider assembly106 flying above the surface 104 of a rotating disk 102 is shown. Theread/write head slider assembly 106 is affixed to an associatedsuspension 108 and test arm 110 having, for example, an acousticemission (AE) sensor 112 associated therewith.

In conventional applications for use with glide avalanche (GA) disks, asensor is mounted on the glide on the glide head itself. As illustratedherein, the AE sensor 112 is mounted remotely from the head and may bemounted directly on the test arm adjoining the attachment of thesuspension. In a particular implementation of the system and method ofthe present invention, low flyers may be detected in accordance with thetechnique disclosed herein using, inter alia, a contact start stop (CSS)tester such as the Olympus tester produced by the Center for Tribology,Inc. (CETR), Campbell, Calif.

With reference additionally now to FIG. 2B, an enlarged view of aread/write head slider assembly 106 as shown in the preceding figure isdepicted illustrative of the head slider assembly 106 flying below theheight of predetermined height laser bumps 114 formed on the surface 104of a DET test disk 102 media. Like structure to that previouslydescribed with respect to FIG. 2A is like numbered and the foregoingdescription applies to this structure and/or these components. Inoperation, contact between the head slider assembly 106 and one or moreof the bumps 114 will serve to generate an AE signal by means of sensor112 (FIG. 2A) which can be monitored to enable screening of low flyersin accordance with a system and method of an embodiment of the presentinvention as disclosed herein.

With reference additionally now to FIGS. 3A, 3B and 3C, graphicalrepresentations of exemplary fly height distributions at mean flyheights (FH) of substantially 10.0 nm (0.40 micro-inches; FIG. 3A), 8.0nm (0.32 micro-inches; FIG. 3B) and 6.5 nm (0.23 micro-inches FIG. 3C)are shown. These figures illustrate fly height and glide avalanche (GA)ranges at population frequencies of between 0.0 and 2000 wherein the lowflying heads are seen to touch as the mean fly height is lowered.

The range of mean fly height applicable to a specific embodiment of thepresent invention disclosed herein may be from 6.4 nm (0.25micro-inches) to 12.7 nm (0.50 micro-inches). A possible cut-off rangefor low flyers would then be, for example, from mean fly height—1.5sigma (6.7% of normal distribution) to mean fly height—3 sigma (0.13% ofnormal distribution). In accordance with the Case 2 scenario of FIG. 3Bin particular, a mean fly height of 8.0 nm (0.32 micro-inches) isillustrated with a sigma of 1.0 nm. With a cut-off threshold for lowflyers set at 8.0 nm—2 sigma, this equates to 6.0 nm (0.24micro-inches). As such, this would remove approximately 2.3% of thepopulation of heads (given a normal distribution) and improve theoverall head/disk reliability.

With reference additionally now to FIG. 4, a simplified productionprocess 300 flow chart in accordance with an embodiment of the presentinvention is shown illustrating the use of an on-line AE tester toenable the screening of low flying heads prior to dynamic electricaltest (DET). The process 300 includes the introduction of head gimbalassemblies (HGAs) to the DET test stand at step 302, and introduction ofbump disk samples to the DET test stand at step 306, for AE testing atstep 304. The HGAs passing through the AE tester step 304 are thensubjected to a DET test at step 308, following which they are furtherassembled into head stack assemblies (HSAs) at step 310 and then intohead disk assemblies (HDAs) at step 312.

In other embodiments (not shown), the AE tester step 304 is performedafter the head gimbal assemblies are assembled into head stackassemblies at 310 (i.e., the AE testing using the disk bumps may beperformed upon one or more of the disks in a head stack rather than oneach head gimbal assembly). In other words, a “group” of HGAs may betested individually prior to assembly into an HSA or a “group” of HGAsmay be assembled into an HAS and then, one or more of the HGAs in theHSA may be tested with an AE tester at step 304.

In the production of HGAs, the AE test step 304 and DET test step 308may conveniently be combined (as indicated by the dashed-line box but itshould be noted that the DET test step 308 is not required to practicethe invention) and accomplished on the same DET test stand in accordancewith the representative embodiment of the present invention shown, interalia, to reduce overall cycle time. Further, the method may beimplemented utilizing certain electrical testers available from GuzikTechnical Enterprises, Mountain View, Calif., with an AE sensor (e.g.sensor 112 of FIG. 2A) to detect head/disk interactions.

With reference additionally now to FIG. 5, a graphical illustration ofan exemplary minimum head fly height cut off (in micro inches) versusbump height (in Angstroms) is shown in a representative implementationof the system and method of the present invention. In this example, thebumps are assumed to have negligible effect on fly height except at thelocation of each bump.

A disk incorporating specific bumps (whether formed by laser orotherwise) for use in screening low flyers may include bumps or otherprotrusions similar to the laser textured bumps on the disk landingzone. The height of the bumps, the bump density (i.e., the number ofbumps on the disk surface per unit area), and/or their radial andcircumferential spacing may be optimized for screening out low flyers.For example, assuming negligible bump effect on fly height as shown, theheight of the bumps will be close to the actual flying height of slidersthat fly just high enough to not be rejected (i.e. screened out) asflying too low. In an embodiment of the present invention this maycorrelate with the negative three sigma (3σ) threshold in a fly heightdistribution of a group of manufactured sliders built into HGAs. In theexemplary case of a 0.24 micro-inch flying height, the bump height wouldthen be close to 6.0 nm or possibly higher. However, the AE signalstrength depends on bump characteristics including bump height, bumpdensity on the disk, bump height relative to fly height, and/or otherbump characteristics. Higher bump density can be used and a bump heightgreater then 6.0 nm may be needed in order to compensate for the effectof the bumps on fly height as the heads will tend to fly higher thannormal. The actual bump height is a matter of design choice and may bedetermined by the calibration of the tester with a range of heads withknown fly heights and corresponding bump test disks with a range of bumpheights.

With reference additionally now to FIG. 6, a functional block diagram ofan integrated head tester system 500 in accordance with an embodiment ofthe present invention is shown which is illustrative of key componentsfor DET measurements and screening of low flying heads utilizing, forexample, AE signal detection. As previously described with respect toFIG. 2A, a read/write head slider assembly 106 may be affixed to anassociated suspension 108 and test arm 110 having, for example, anacoustic emission (AE) sensor 112 associated therewith. The head sliderassembly 106 flies over the surface 104 of test disks which are used toscreen out low flyers. These disks have incorporated special laser bumpheights at the inner diameter (ID), middle diameter (MD) and outerdiameter (OD). These bumps may also conveniently be provided on DET testdisks such that DET testing will immediately follow this test on thesame disk as illustrated at step 308 in the process 300 of FIG. 4. Headsthat fly below the minimum fly height specification will physicallytouch the bumps and cause a higher AE signal from the sensor 112. Thiswould result in the rejection of the head before or concurrent with DETtesting.

As shown, output from the sensor 112 may be applied through an AEpreamplifier 502 to an AE analyzer 504. In addition, output from theread/write head itself may be supplied to a read/write (R/W) signalpreamplifier 506 to a corresponding R/W signal analyzer 508. Output fromthe AE analyzer 504 and R/W signal analyzer 508 may be furnished to acomputer 510 which operatively enables a spin stand controller 512. Thespin stand controller 512 provides control signals to a spin stand 514,which serves to rotate the test disk in a controlled manner, and mayalso provide control signals to a course servo positioner 516 and amicropositioner 518 which control the positioning of the read/write headradially over the disk surface 104.

In operation, the laser bump height, diameter and spacing are optimizedto produce a higher AE signal from the sensor 112 if the head fliesbelow a certain minimum fly height (e.g. 0.25 micro-inches) as shown inthe graph of FIG. 5. As may be required, a special batch of test headsmay be made to check the AE sensitivity to fly height. In representativeapplications, the heads can fly in the range of 0.40 to 0.20micro-inches. As an example, a head may be selected that flies at 0.25micro-inches and a corresponding bump height and spacing can then alsobe selected that will result in a strong AE signal if it fliessignificantly below that height.

While there have been described above the principles of the presentinvention in conjunction with specific exemplary test equipment,methodologies and the like, it is to be clearly understood that theforegoing description is made only by way of example and not as alimitation to the scope of the invention. Particularly, it is recognizedthat the teachings of the foregoing disclosure will suggest othermodifications to those persons skilled in the relevant art.

1. A system for manufacturing a group of head gimbal assemblies (HGAs),said system comprising: a first test disk region including a magneticlayer; a second test disk region having a plurality of bumps extendingtherefrom to a predetermined height greater than 2 nm and less than 12nm; a rotation mechanism rotating either or both test disk regions; apositioning mount positioning a head of an HGA selected from the groupadjacent the second test disk region; write/read electronics causing thepositioned head to write data to and read data from the first test diskregion; and a sensor for sensing an interaction of the positioned headwith one or more of the plurality of bumps.
 2. The system of claim 1,wherein the first and second test disk regions are on one disk.
 3. Thesystem of claim 1, wherein the first and second test disk regions areprovided on first and second disks, respectively.
 4. The system of claim1, wherein the predetermined height is greater than a glide avalancheheight that is characteristic of a lot of product disks to be usedtogether with said group of head disk assemblies in later manufacture ofa quantity of disk drives.
 5. The system of claim 1, wherein the secondtest disk region is formed by focusing a laser beam on the first testdisk region at selected locations to form the plurality of bumps.
 6. Thesystem of claim 1, wherein the sensor includes an acoustic emissionsensor affixed to a test arm associated with the head gimbal assemblyselected from the group.